Unit fuel injectors of the type that are individually associated with each of the cylinders in an internal combustion engine may be closed nozzle or open nozzle in construction. Closed nozzle injectors employ a pressure sensitive valve mechanism that opens a metering chamber only during injection, while open nozzle injectors include a metering chamber that is kept open to the engine cylinder. Open nozzle fuel injectors are preferred for high pressure injector applications in diesel engines. Closed nozzle injectors are substantially more complex structures than open nozzle injectors, function on different operational principles from an open nozzle injector, and cost more to produce. Although the plungers and associated structures of both types of fuel injectors are susceptible to wear, the plungers and plunger seats of open nozzle injectors are particularly subject to wear and associated performance degradation.
In some diesel fuel injector applications the engine duty cycles include prolonged motoring conditions. When the engine is operating under such motoring conditions, little or even no fuel may be present in the injector metering chamber. During prolonged motoring conditions the injector plunger, which is driven by the rotating camshaft, continues to operate. As a result, the temperature inside the metering chamber can become quite high. After a period of time, the plunger and seat, in particular, exhibit varying degrees of wear, which is a combination of material fatigue, deformation and wear. The performance of the injector is inefficient and even ineffective because of this wear. Engine performance, particularly the output and the completeness of combustion, suffers when injector performance is degraded. Moreover, if the wear of the injector plunger assembly is extensive, failure of the injector is likely. Frequent replacement of the injector components increases the cost of engine operation.
The problem of unit fuel injector plunger assembly wear caused by prolonged motoring conditions in a diesel engine has not been addressed by the prior art. U.S. Pat. No. 4,266,729 to Kulke et. al. proposes modifying a closed tip fuel injector by making the nozzle needle tip and disc containing the ejection opening from a corrosion-resistant material, such as steel, ceramic or industrial glass, to prevent the constriction of the ejection opening from corrosion. However, Kulke et. al. does not suggest modifying the plunger itself or other plunger components in a closed nozzle or an open nozzle fuel injector to make them resistant to the material fatigue, deformation and wear caused by prolonged motoring of the engine.
It has been suggested to form fuel injector components and other engine components frown ceramics, in part to enhance their wear-resistance. In U.S. Pat. No. 4,991,771, Porchet et. al. disclose the use of a pair of ceramic rings on each side of a nozzle opening in a fuel injection valve to reduce corrosion or erosion in the valve. U.S. Pat. No. 4,592,506 to Capes et. al. discloses an atomizing nozzle assembly with a cone-shaped deflector core and a nozzle rim made of a wear-resistant ceramic material. U.S. Pat. Nos. 4,794,894 to Gill; 4,806,040 to Gill et al and 4,848,286 to Bentz disclose, respectively, ceramic tipped pivot rods and a ceramic ball and socket joint for use in internal combustion engines. None of the foregoing patents suggests prolonging unit fuel injector life or preventing injector component wear in a fuel injector when a diesel engine is repeatedly subjected to the prolonged motoring conditions which occur when the engine functions as a vehicle retarder, such as during compression brake operation.
During compression brake operation, fuel metering is suspended while the injector continues to operate without the cooling and lubricating benefit of the fuel. Efforts have been undertaken to solve the problem of plunger and nozzle wear caused by prolonged motoring by the assignee of the present invention. Different materials, primarily different types of steel, were used to form the injector nozzle and plunger to enhance their wear-resistance. Additionally, heat treatments, coatings, such as TiN, carburizing, and nitriding were employed for this purpose. Lower wear combinations of plunger and nozzle materials were identified which were effective in reducing motoring wear. However, the wear reduction thus achieved was not sufficient to improve engine performance, particularly in the areas of power output, fuel economy and emissions, over a wide range of operating conditions.
The prior art, therefore, has failed to provide a means of improving engine performance while eliminating excessive fuel injector component wear, especially plunger and nozzle seat wear in a fuel injector, particularly an open nozzle unit fuel injector in a diesel engine having a duty cycle that includes prolonged motoring. Consequently, there is a need for a wear-resistant plunger assembly for a unit fuel injector capable of resisting material fatigue, deformation and wear when the engine is repeatedly subjected to prolonged periods of motoring so that engine performance is improved over a wide range of operating conditions.